ND grads are interesting and their use depends on your take on photography. Their effect can generally be mimicked in lightroom or photoshop with a combination of either multiple exposures, or simple under exposing then lifting the shadow information. I generally accept that in camera, I can liberate about 3-4 stops of shadow information, and as such exposing the sky mostly correctly, bumping the total exposure of the image and then adding an ND grad in lightroom works well enough....
... However
I do like doing things optically, and as a result I went for some Cokin square filters and holder as they allow me to use a versatile set of lenses with the same filters. It does make a difference to achieve the shot optically, as it helps control the exposure from the start, meaning less you have to "rescue" in post. But as I said above, this one can be faked.
What can't be faked, is a standard ND filter. If you're shooting at sunset, and you're at F11, trying to get a 3-4 second exposure to make the water all dreamy, and even at ISO 100 the best you can get is 0.5 seconds, then you're up against the wall and theres no number of tricks can slow the camera down. That's where a Solid ND comes in, and I find them hugely useful to really slow down the shot in conditions where theres too much light.
Like this shot:
Crammon Sunset 5.jpg by
David, on Flickr
However a fake ND can work very well, and doesn't demand a physical filter, like this shot:
Torn by
David, on Flickr
Only you can decide what works for you
